[OFDM]
In data transmission by OFDM (Orthogonal Frequency Division Multiplexing), a large number of orthogonal subcarriers are used in the transmission band, and data is allocated to the amplitude and the phase of each of the subcarriers. Data of each symbol called an OFDM symbol is transmitted. At the time of transmission, an IFFT (Inverse Fast Fourier Transform) is performed on each OFDM symbol.
FIG. 1 is a diagram showing OFDM symbols. An OFDM symbol is normally formed with an effective symbol that is a signal interval in which an IFFT is performed at the time of transmission, and a guard interval (GI) formed by copying the waveform of part of the latter half of the effective symbol and placing the copy at the top of the effective symbol.
As the guard interval is formed at the top of the OFDM symbol, resistance to multipathing can be increased. Such OFDM symbols constitute one OFDM transmission frame.
[MISO of DVB-T2]
According to the second-generation digital terrestrial broadcasting standards in Europe (DVB-T2), data transmission by MISO (Multi Input, Single Output) can be performed. For example, data is transmitted by using two antennas on the transmission side. Accordingly, diversity can be generated by a combination of signals transmitted from the two antennas, and reception performance can be improved. Data transmission by MISO is particularly effective in an intensive multipathing environment.
FIG. 2 is a diagram showing data transmission by MISO according to DVB-T2.
As shown in FIG. 2, a MISO transmitter has the two antennas of an antenna 1 (Tx1) and an antenna 2 (Tx2), and a MISO receiver has one antenna (Rx1). Two signals Sa and Sb, which form an Alamouti pair, are input to the MISO transmitter.
The MISO transmitter performs Alamouti coding on Sa and Sb, and transmits Sa and Sb in this order from Tx1. Meanwhile, the MISO transmitter transmits (Sa)*, which is a complex conjugate of Sa, and −(Sb)*, which has the inverted sign, from Tx2 in the order of −(Sb)* to (Sa)*.
The MISO receiver estimates channel characteristics H11, H12, H21, and H22, and performs Alamouti decoding on received signals Ra and Rb, to obtain the transmitted signals Sa and Sb (Sa′ and Sb′). Where the time to transmit Sa from Tx1 and −(Sb)* from Tx2 is time t1, and the time to transmit Sb from Tx1 and (Sa)* from Tx2 is time t2, H11 represents the channel characteristics between Tx1 and Rx1 at time t1, and serve as a weight on Sa. Likewise, H12 represents the channel characteristics between Tx1 and Rx1 at time t2, and serve as a weight on Sb. H21 represents the channel characteristics between Tx2 and Rx1 at time t1, and serve as a weight on −(Sb)*, and H22 represents the channel characteristics between Tx2 and Rx1 at time t2, and serve as a weight on (Sa)*.
At the MISO receiver, the received signals Ra and Rb are expressed by the following equations (1) and (2).Ra=H11Sa−H21Sb*  [Mathematical Formula 1]Rb=H12Sb−H22Sa*  [Mathematical Formula 2]
The equations (1) and (2) are transformed into a determinant as shown in the following equation (3).
                    [                  Mathematical          ⁢                                          ⁢          Formula          ⁢                                          ⁢          3                ]                                                                      [                                                                      R                  a                                                                                                      R                  b                  *                                                              ]                =                              [                                                                                H                    11                                                                                        -                                          H                      21                                                                                                                                        H                    22                    *                                                                                        H                    12                    *                                                                        ]                    ⁡                      [                                                                                S                    a                                                                                                                    S                    b                    *                                                                        ]                                              (        3        )            
Where a matrix R represents the received signals, a matrix H represents the channel characteristics, and a matrix S represents the transmitted signals, the equation (3) is expressed by the equation (4) shown below. The matrixes R, H, and S are expressed by the equations (5), (6), and (7), respectively.
                    R        =        HS                            [                  Mathematical          ⁢                                          ⁢          Formula          ⁢                                          ⁢          4                ]                                [                  Mathematical          ⁢                                          ⁢          Formula          ⁢                                          ⁢          5                ]                                                            R        =                  [                                                                      R                  a                                                                                                      R                  b                  *                                                              ]                                    (        5        )                                [                  Mathematical          ⁢                                          ⁢          Formula          ⁢                                          ⁢          6                ]                                                            H        =                  [                                                                      H                  11                                                                              -                                      H                    21                                                                                                                        H                  22                  *                                                                              H                  12                  *                                                              ]                                    (        6        )                                [                  Mathematical          ⁢                                          ⁢          Formula          ⁢                                          ⁢          7                ]                                                            S        =                  [                                                                      S                  a                                                                                                      S                  b                  *                                                              ]                                    (        7        )            
The Alamouti decoding by the MISO receiver is expressed by the equation (8) shown below. S′ is the matrix representing the signals obtained after the Alamouti decoding.
                    [                  Mathematical          ⁢                                          ⁢          Formula          ⁢                                          ⁢          8                ]                                                                                                                S                ′                            =                            ⁢                                                H                                      -                    1                                                  ⁢                R                                                                                        =                            ⁢                                                H                                      -                    1                                                  ⁢                HS                                                                                        =                            ⁢              S                                                          (        8        )            
In the equation (8), to obtain the matrix S representing the transmitted signals from the matrix R representing the received signals, the matrix R is multiplied by the inverse matrix of the matrix H representing the channel characteristics. If there is no noise, the matrix S is exactly the same as the matrix S′. The matrix S′ is expressed by the following equation (9).
                    [                  Mathematical          ⁢                                          ⁢          Formula          ⁢                                          ⁢          9                ]                                                                      S          ′                =                  [                                                                      S                  a                  ′                                                                                                      S                  b                                      *                    ′                                                                                ]                                    (        9        )            
The MISO receiver outputs Sa′ and Sb′, which are determined in the above described manner.
FIG. 3 is a diagram showing an example of arrangement of SP (Scattered Pilot) signals in a case where data transmission is performed by MISO. In FIG. 3, the abscissa axis indicates carrier number, and the ordinate axis indicates symbol number.
SP signals are known signals that are used in estimating channel characteristics. In the digital terrestrial broadcasting standards in Europe (DVB-T) and the digital terrestrial broadcasting standards in Japan (ISDB-T), arrangement of SP signals is uniquely set. In DVB-T2, on the other hand, eight Pilot Patterns (PP) are defined. A MISO receiver of DVB-T2 determines a PP based on information contained in received signals, and estimates channel characteristics by using SP signals. The SP arrangement shown in FIG. 3 is the arrangement in the case of PP2.
When data transmission is performed by MISO, SP signals are transmitted as Sum SP signals or Diff SP signals, as shown in FIG. 3.
A Sum SP signal is a SP signal for a symbol having an even number allotted thereto (the symbol number being an even number). Sum SP signals are transmitted as SP signals having polarities not to be changed (hereinafter referred to as normal SP signals) from Tx1 and Tx2, and are combined at the MISO receiver.
A Diff SP signal is a SP signal for a symbol having an odd number allotted thereto (the symbol number being an odd number). Diff SP signals are transmitted as normal SP signals from Tx1 and as SP signals having polarities reversed (hereinafter referred to as inverted SP signals) from Tx2, and are subjected subtractions at the MISO receiver. In a state where a polarity is reversed, a symmetry with respect to the point of origin in the I-Q plane is observed.
The MISO receiver performs interpolation on the Sum SP signals and the Diff SP signals in the temporal direction and the frequency direction, and estimates the channel characteristics of all the carriers.
[Signaling of DVB-T2]
According to DVB-T2, frames called T2 frames are defined, and data is transmitted by the T2 frame.
Each T2 frame contains two kinds of preamble signals called P1 and P2, and those preamble signals contain information necessary for operations such as OFDM signal demodulation.
FIG. 4 is a diagram showing the frame structure of a T2 frame. As shown in FIG. 4, one T2 frame includes a P1 symbol, P2 symbols, and data symbols (Normal or FC).
The P1 symbol is the symbol for transmitting P1 Signaling, and contains the following information a through d.
a. Frame identification
b. Transmission method
c. FFT size
d. Partial GI length
The frame identification indicates whether the transmission frame is a T2 frame or a FEF (Future Extension Frame). The transmission method indicates whether the transmission method is SISO (Single Input, Single Output) or MISO (Multiple Input, Single Output). The FFT size indicates the number of points in one IFFT operation on the transmission side. The partial GI length indicates to which group the GI length being used in the symbol transmission belongs, with seven types of GI lengths being divided into two groups.
To determine whether the transmission method used for transmitted signals is SISO or MISO, the MISO receiver should decode P1 Signaling. Although the above described information a through d is subjected to Signaling in an overlapping manner in the P2 symbols, the above described information a through d of P1 Signaling is necessary for decoding L1PRE Signaling and L1POST Signaling of the P2 symbols.